1. Field of the Invention
The invention relates to a method for increasing the accuracy of measurement of alignment systems with several measurement planes which are located in succession in the beam path. The use of at least two measurement planes makes it possible to determine not only the incidence point of a light beam, but also the direction from which this light beam strikes the detector with these measurement planes.
2. Description of Related Art
These measurement systems are known from Patent Application DE 38 14 466 A1 and corresponding U S. Pat. No. 6,356,348 B1. Since hardly any transparent detectors are available for these measurement systems, beam splitters in the form of partially transparent mirrors are used, whose coating is often applied to the diagonal surface of a glass cube. These cubes are formed by cementing two prisms with the cross section of an equilateral-right triangle. These beam splitter cubes are, however, expensive to produce. Therefore, often only a glass plate is partially mirrored. Often, lenses are placed in the housing of the alignment detector. Detectors are photosensitive diodes (PSD) which can be read out one-dimensionally or two-dimensionally. CMOS or CCD-based arrays are also used. Of course, more degrees of freedom of misalignments can be computed with detectors which can be read out two-dimensionally.
However, the accuracy of these measurement systems is limited by systematic imaging errors, especially the aperture aberration and the resulting errors, such as spherical aberration, astigmatism and coma, and by nonsystematic imaging errors, such as reflections on optically active surfaces, production tolerances and shape faults. These imaging errors have an especially serious effect if the light beam does not run near the optical axis of the system, but far away from it, or if the light beam includes an angle relative to the optical axis of the system. If a larger detector is used to be able to measure larger offsets, the effect of imaging errors increases.